The present invention relates to a light emitting device and a light emitting system utilizing the same.
In general, a self-luminous light emitting element used in a display device includes a field emission element and an electro-luminescence element (EL element). The EL element can be sorted into an organic EL element having a light emitting layer formed of an organic material and an inorganic EL element having a light emitting layer formed of an inorganic material.
The organic EL element comprises an anode, a cathode, and a very thin organic EL layer sandwiched between two electrodes of the anode and the cathode and formed of an organic light emitting compound. If a voltage is applied between the anode and the cathode, holes and electrons are injected to the organic EL layer from the anode and the cathode, respectively, so that the holes and the electrons are recombined in the organic EL layer. Molecules in the organic light emitting compound constituting the organic EL layer are excited by an energy generated when the holes and the electrons are recombined. In the process in which the molecules thus excited de-excite to a ground state, a light emitting phenomenon occurs. The organic EL element is a light emitting element utilizing this light emitting phenomenon.
The organic EL layer has a single-layer structure or multi-layer structure, which includes an organic layer called a light emitting layer, for emitting light by hole-electron recombination, and at least one of an organic layer called a hole injection layer having the property easy to inject the holes but difficult to transfer the electrons, and another organic layer called an electron injection layer having the property easy to inject the electrons but difficult to transfer the holes.
Recently, the organic EL element is actively researched, and is becoming reduced in practice. This is an element having a fundamental structure which is formed by depositing a hole injection material such as triphenyl diamine (TPD) in the form of a thin film on a transparent electrode (hole injection electrode, namely, anode) such as a tin-doped indium (ITO), further depositing thereon a fluorescent substance such as aluminum quinolinol complex (Alq3) as a light emitting layer, and forming a metal electrode (electron injection electrode, namely, cathode) such as AgMg, having a small work function. With a voltage of about 10V, an extremely high luminance can be obtained on the order of a few hundred to a few ten thousand cd/m2, and therefore, attention is focused onto the organic EL element as electrical equipments and displays in in-home electric goods, automobiles, motorcycles and aircrafts.
The above mentioned organic EL element has a structure in which for example, an organic layer such as a light emitting layer is sandwiched between a scan (common line) electrode functioning as the electron injection electrode and a data (segment line) electrode functioning as the hole injection electrode (transparent electrode), this assembly being formed on a transparent (glass) substrate. The organic EL element constituted as a display, can be divided into a matrix display in which dots are displayed by the scan electrodes and the data electrodes arranged to form a matrix so that information such as an image and characters is displayed by an aggregation of dots (pixels), and a segment display in which display elements having predetermined shapes and sizes are arranged independently of each other.
In the segment display, a static driving method can be adopted which independently drives each display segment. In the matrix display, on the other hand, a dynamic drive method is ordinarily adopted to drive each scan line and each data line in a time division manner.
In a full-color light emitting display using the above mentioned electro-luminescence (EL) element, a problem is encountered in which there occur an unevenness of luminance and a deterioration in a color balance in the display.
The luminance in the organic EL element greatly depends upon each of the layers constituting the organic EL element, particularly, the film thickness of the light emitting layer. Because, the smaller the region in which holes and electrons are recombined is, the larger the luminous efficiency becomes. Therefore, if there is unevenness in the film thickness of the organic layer included in the organic EL element, unevenness occurs in luminance and in color generation. A method for directly forming an organic layer pattern by use of an ink jet is known as a method for forming the organic layer. In this method, however, it is difficult to control the film thickness of each pattern, with the result that luminance unevenness and color generation unevenness are apt to occur.
In addition, in the full-color light emitting display using the organic EL element, the luminance and the color are adjusted by controlling the proportion in light emission of three colors of red, blue and green. This adjusting method includes a method for adjusting the balance in the number of actually light emitting elements of each color and a method for adjusting the balance in the light emitting time of the actually light emitting elements of each color. Here, however, it is a problem that the luminance of the organic EL element changes with the elapse of the light emitting time. In addition, the change with time in the luminance is different from one emitted color to another. Therefore, if a certain time of use has elapsed, the color balance becomes deteriorated.
In order to solve this problem, Japanese Patent Application Pre-examination Publication No. JP-A-59-55487 proposes an electro-luminescence display element (EL display element) in which an organic EL element is associated with a sensor part having an output electricity amount varying dependently upon the luminance of the light emitted from the organic EL element, for the purpose of controlling a drive voltage applied to the organic EL element, so that if the luminance of the organic EL element decreases, the drive voltage level is increased, thereby to substantially maintain the display effect at a constant. As an example of the sensor part capable of detecting the luminance of the light emitted from the element so as to change the drive voltage, FIG. 5 and FIG. 6 of JP-A-59-55487 illustrate an example in which the sensor is located on one plane of a substrate having an opposite plane on which the organic EL element is provided, and FIG. 8 of JP-A-59-55487 illustrates an example in which the sensor is located between the substrate and the organic EL element. In addition, FIGS. 2 to 4 of JP-A-59-55487 illustrate an example in which the sensor part capable of detecting the luminance of the light emitted from the element so as to change the drive voltage, is located at a side of the organic EL element.
However, in the case that the sensor part is provided under the organic EL element, it become necessary to form the organic EL element after the sensor part is formed, so that the characteristics of the sensor part is in some cases deteriorated for the influence of the process for forming the organic EL element, or the surface condition of the underlying layer on which the organic EL element is to be formed becomes concavo-convex because of existence of the sensor part, with the result that the characteristics of the organic EL element formed on such an underlying layer is deteriorated in some cases. On the other hand, in the case that the sensor part is located at the side of the organic EL element, since the light emitted from the organic EL element is not effectively inputted on the sensor part, it is difficult to sensitively detect the strength of the light emitted from the organic EL element.
In the case that the sensor part is located on one plane of a substrate having an opposite plane on which the organic EL element is provided, since the organic EL element is divided into a substrate surface light emitting type configured to emit the light through the substrate and a film surface light emitting type configured to emit the light through no intermediary of the substrate, this structure can applied to the former type because this structure needs to locate the substrate between the light emitting element and the sensor part. In addition, when the substrate is located between the light emitting element and the sensor part, it becomes necessary to turn the substrate in the way of the manufacturing process, and therefore, the manufacturing process becomes complicated. Furthermore, since the light emitting element and the sensor are provided on opposite surfaces of the substrate, respectively, the alignment between the light emitting element and the sensor becomes difficult. Specifically, it is necessary to previously provide alignment marks of element patterning on the opposite surfaces of the substrate, respectively, and it is difficult to precisely align the light emitting element and the sensor with each other, because of the thickness of the substrate. The smaller the size of the light emitting element is, the more the required alignment becomes precise, and therefore, it becomes important to elevate the required alignment.
Furthermore, Japanese Patent Application Pre-examination Publication No. JP-A-04-190326 discloses a liquid crystal display with a back light in which a device (phototransistor) for detecting the luminance of the EL element is located at a position irradiated with a portion of the light emitted from the EL element when the EL element is energized, specifically, at a position slightly deviated from a position between the EL element and a liquid crystal display part (FIG. 1). Also in this case, however, since the light emitted from the EL element is not effectively inputted on the phototransistor, it is difficult to sensitively detect the luminance of the EL element. In addition, since this system is such that the amount of the light emitted from the EL element is detected by one phototransistor so that the luminance of the total of the EL elements is controlled by the result of the detection, this system can elongate the service life of the EL light emitting device, but cannot minimize the unevenness in luminance and the deterioration in the color balance.
Japanese Utility Model Application Pre-examination Publication No. JU-A-62-158792 discloses an EL light emitting device in which a photodiode of a luminance compensating circuit is located at an end of a light emitting part. In this example, however, since the light emitted from the EL element is not efficiently inputted to the photodiode, it is difficult to sensitively detect the luminance of the EL element. In addition, since this system is such that the amount of the light emitted from the EL element is detected by one photodiode so that the luminance of the total of the EL elements is controlled by the result of the detection, this system can elongate the service life of the EL light emitting device, but cannot minimize the unevenness in luminance and the deterioration in the color balance.
Japanese Utility Model Application Pre-examination Publication No. JU-A-62-111199 and Japanese Patent Application Pre-examination Publication No. JP-A-04-254889 disclose an EL display device in which, in addition to pixels, a reference pixel (detecting electro-luminescence element) is provided at a position out of a display region, and the strength of the light emitted from the reference pixel is detected. In order to prevent the unevenness in luminance and the deterioration in the color balance by the reference pixel, it is necessary to actually form many reference pixels, and this is disadvantageous in view of productivity and others. In addition, it is possible to more effectively prevent the unevenness in luminance and the deterioration in the color balance by optimizing the amount of emitted light on the basis of the luminance of the light emitted from the light emitting element itself, rather than by optimizing the amount of emitted light or the light emitting time on the basis of the luminance of the light emitted from the reference pixel.